As was mentioned, it is just an ordinary envelope that one could buy in any store that sold paper and envelopes just like today where you can go to Staples or some such store and buy all of the plain envelopes you like and pay more for the fancy ones and less for the plain ones. The cheaper the paper, the less likely it was to have a watermark and optimizing profits is what most companies and retail stores try to do. There is no significance to it not having a watermark other than to indicate the grade of the paper used by some company that made this envelope was on the lower end of the scale.
This cover was mailed shortly before the end of the war which happened on November 11, 1918. Like every cover mailed by an enlisted man during the war it was censored by his commanding officer - usually a lieutenant. Officers censored their own mail. During the war, active duty soldiers and sailors could mail letters with no postage which is why there is no stamp on this cover. This particular one was sent by a private in the U.S. Army 26th Engineers. There is a book that was published on this unit:
http://www.genealogycenter.info/mil...ngineers.php Collecting war time censored mail is a big and fun collecting interest area. There are a number of books and catalogs on the subject and the number of different varieties is enormous making it a never ending quest for a collector. In addition to the different military post office markings, many collectors also like to collect cards and covers from the more romantic and interesting soldiers and sailors. For example, this one came from a private in an Engineering unit which is not as sought after as something like an aviation unit or from a female nurse, or a tank unit, or a marine corps unit, or a submarine instead of a ship, etc.